Power converter are widely used to provide required voltages and load currents especially in environments that have higher concentrations of telecommunications or computer equipment. These power converters are often required to provide higher levels of output power for a small physical volume. This requirement dictates that the power converters be high power density devices. These higher power density concentrations also dictate that the power converter operate with as high an efficiency as possible to minimize the converter's heat generation.
Flyback converter is one of the widely used power supply topologies due to its high converting efficiency, low cost, small size and less components. Therefore, the flyback converter has been widely used as a power supply for a variety of electronic devices. In general, a flyback converter is used in the DC-DC conversion for driving a coupled load with transformer-isolated between input and output, an input rectifier circuit can be used in AC-DC application. A conventional flyback converter includes mainly an input circuit, an output circuit, a transformer, and an optical coupler. An input circuit connects to an input voltage and contains mainly two parts, a transistor for switching and a controller for regulating PWM (pulse width modulation). The transistor connects to primary side winding of the transformer at one end, and also connects to the output of the controller at the other end. The input FB end of the controller connects to one end of the optical coupler. At the system's output side, an output circuit connects to a secondary-side winding of the transformer. The output voltage Vout connects in parallel to one end of the optical coupler, isolating the input circuit from the output circuit, and conveying the output voltage back to the controller. Consequently, the controller is able to output a stable voltage by controlling the on-off states of the transformer.
The above-mentioned prior art utilized the feedback control function of an optical coupler to regulate the output voltage. The physical characteristic of an optical coupler unavoidably affects the stability and durability of the system. For example, the coupling efficiency of an optical coupler reflects the accuracy of an output voltage. In further, extra electric components are required to reduce the un-stability of the system, thus adding extra cost and requiring more space.
To solve the above issues, a self-excited flyback converter disclosed in U.S. Pat. No. 7,835,163, as illustrated in FIG. 1A, was proposed. The flyback converter converts an input voltage Vin to an output voltage Vout. It utilized a controller 21 for secondary side regulation at a secondary side of a transformer 11 to control a switching device 13 at a primary side of the transformer 11, to achieve a better regulation.
As illustrated in FIG. 1A, the flyback converter includes an input circuit 1, a transformer 11, and an output circuit 2. The input circuit 1 includes one or more than one switching devices 13, an input end and an output end; the input end of the input circuit 1 connects to an input voltage Vin, the output end of the input circuit 1 connects to a transformer 11. The transformer 11 contains a primary winding and a secondary winding, the primary winding connects to a switching device 13, which regulates the on-off states of the switching device 13 according to the output voltage Vout variation of the secondary winding. Referring to FIG. 1A, a circuit configuration between a first control terminal CTL1 and the second control terminal CTL2 in the controller 21 may be configured as one of a resistor load connection, a short circuit and an open circuit. When the output voltage Vout is higher than a target voltage, the controller 21 is operable to set the circuit configuration to the resistor connection for a period of time so as to stop the switching device 13 from being conducted. Therefore, the self-excited conversion from the input voltage Vin to the output voltage Vout stops, such that the output voltage Vout is prevented from rising. When the output voltage Vout is lower than a target voltage, the controller 21 is operable to set the circuit configuration to the short circuit for a period of time to enable the switching device 13 to conduct. Therefore, the self-excited conversion starts for raising the output voltage Vout. Otherwise, the controller 21 is operable to set the circuit configuration to the open circuit so as to avoid interfering with the operation of the conventional switching power converting apparatus.
The following are some of the drawbacks of the conventional switching power converting apparatus:
(1). It is hard for a designer to determine a resistance value between the first control terminal CTL1 and the second control terminal CTL2 of the controller 21 which has the circuit configuration been the resistor load connection.
(2). The resistance value between the first control terminal CTL1 and the second control terminal CTL2 of the controller 21 may vary during fabrication process of the controller 21, and may drift along with temperature variation, etc.
As illustrated in FIG. 1A, the energy input from the input circuit 1 has been converted into a output voltage Vout of the output terminal of the output circuit 2 through the transformer 11. The output circuit 2 further includes a controller 21 for detecting variation in voltages between output voltage (Vout) and a predetermined voltage, and send back the detected voltage variation to the primary winding as a decision-making feedback the voltage variations. The primary winding then reacts to voltage variations by controlling the on/off states of switching device in PWM to stabilize the output voltages.
In the past decade, the power consumption in portable electronic devices, such as smart phones or tablet PCs, has largely increased as these devices have increasing size of screen and high performance CPUs. Therefore, the batteries installed in these portable devices have increased storage capacity, more electric power is needed for charging these batteries to accommodate the increased power demands. Conventionally, in these portable products, the Universal Serial Bus (USB) port is often used both as communication port and as a power delivery port to accommodate battery charging. A standard USB 2.0 compliant port may provide a maximum power delivery of 7.5 W (5V at 1.5 A) to recharge the battery of the portable devices. However, this limitation restricts the power converting capability and prolongs the charging time. By increasing the charging voltage, even the charging current is still limited, more power can be transferred to the battery of electronic device and faster charging time can be achieved. As a result, several rapid charge protocols have been proposed and available in the market, to enable a rapid charge mode, it is conventional to use higher output voltages over the USB cable, for example rather than use the default USB output 5V, rapid charging modes have been developed that use higher voltage (7-12 V).
Please referred to FIG. 1B, an AC-DC power supply for rapid charge application with flyback converter circuit topology is illustrated, input circuit on the primary wing includes a transistor switching device 101 connected to the primary winding 103 and a primary side controller 105, the primary controller 105 operates in quasi-resonant mode to provide high efficiency. On the secondary output circuit side, a MOSFET switching device 109 connected to a secondary winding 107 replaces the conventional rectified diode for allowing the current flow to a load, a voltage crossed MOSFET switching device 109 can be monitored by a synchronized rectifier controller 111 to decide when to activate the MOSFET switching device 109, and another secondary side controller 113 is used as a USB power delivery interface for delivering power and handling the communication between a power adapter and portable devices. The secondary side controller 113 used as a USB power delivery interface utilizes one optocoupler 115 for isolating the input circuit and output circuit and sends output voltage signals back to the primary side controller 105.
In summary, the flyback converters mentioned above, each of them has different characteristics, however there still some drawbacks exist for further improvement. For example, the coupling efficiency of an optical coupler reflects the accuracy of an output voltage, extra electric components are required to reduce the un-stability of the system; utilizing a controller on the secondary side to control a switching device on the primary side may cause additional difficulties in the control IC fabrication process for this design involving that a current loop during operation will flow through the interior of the secondary controller.
In order to solve the above mentioned issues, a novel flyback converter with secondary side control is proposed.